In recent years, a reflector has been widely used in the industry field. For example, the reflector is used as a lamp reflector in a backlight unit of a liquid crystal display device, a reflection umbrella for an illuminator, a reflective mirror in the optical system and the like. More specifically, a reflector made of aluminum, silver or white pigments as a reflective material has been widely used. For any of the above applications, the demand for high brilliance and energy saving type products have been voiced. Accordingly, reflectors having much higher reflectance have been in high demand.
As one of methods considered to increase the reflectance, there is a concept of a reflection-increasing film. Usually, a high refractive index thin film layer and a low refractive index thin film layer are laminated at a thickness of λ/4n respectively, whereby such a film can be realized. So, a reflection-increasing film having glass and aluminum as a base has been achieved. Here, the above A refers to a wavelength of the light to be targeted, while n refers to a refractive index of the high refractive index thin film layer or the low refractive index thin film layer at the wavelength.
A reflection-increasing film having silver as a base has been reported in JP1999-2707A, JP2000-180848A, JP1995-191317A, JP2002-55213A and the like. However, the improvement of the reflectance cannot be said to be enough. In particular, there has been reported an example that the improvement of the reflectance in the long wavelength range of not less than 0.7 μm was small.